Multivertebral, particularly dorso-lumbar, osteosynthesis combines the use of screws or hooks connected together by plates or rods.
The use of plates with appropriate recesses allows the screws a certain amount of travel and allows them to slide along an axis. This is useful when fitting screws which diverge in the sagittal plane.
The use of longitudinal connecting members such as rods for example also allows the bone-anchoring elements, for example screws, to slide along the principal axis of the longitudinal connecting member, and allows screws which diverge in the horizontal plane to be brought onto the same antero-posterior line, and this is by virtue of derotation effects imparted on the rods about an apicocaudal axis, that is to say in the horizontal plane.
However, the bending of the rod that this manoeuvring this must be performed between two vertebral segments which are a sufficient distance apart. Furthermore, one or more successive bending operations are performed only in the same frontal plane. This then results in a deformation transposed into another plane, orthogonal to the first.
The adjusting of the pedicle-screws/rod pair may lead to very high stresses in the system before it is definitely locked.
Special-purpose instruments have therefore been conceived.
Pedicle screws in which the threaded shank is extended rearwards have also been developed, so that the descent of the rod as far as the vertebral implantation base of the screw can be guided, segment by segment.
The other benefit of this type of extended pedicle implant is that it allows equal use either of a plate or of a rod.
There are deformations whose radius of curvature may be short, in one or two segments, but, nonetheless, combined in the three planes, sagittal, horizontal and frontal. Simply bending a rod in a single plane, bringing this rod gradually alongside or performing an overall derotation movement, is then no longer suitable.
This is because the reduction by rotation of the rod in the event of bending in two planes is prohibited by the laws of mechanics.
Reduction of a deformation with a large radius, under such conditions, is in three planes, but is not in any way sequential, and can even less be said to be selective.
These short deformations, which can be reduced partially, have to be considered segment by segment and especially plane by plane before any reduction manoeuvre, particularly partial, can be envisaged.
One vertebra which is off-set in isolation in the frontal sagittal and horizontal planes has to be brought into a condition such that it can undergo reduction in just one plane if necessary, or even with a view to be secured as it is to the adjacent segment under no stress other than the stress induced by neutralization.
To meet these requirements, pedicle screws equipped with a "ball joint" system have been designed and developed.
Thus, the head of a screw may be capped by a U-shaped element thus dubbed a "tulip" which acquires mobility about the principal axis of the screw.
The travel obtained makes it possible, within certain limits, to get around the consequences of an angular offset in the horizontal and/or frontal plane of the pedicle alignment.
This being the case, the bending of the rod is no longer a ruse for roughly aligning a poorly frontally aligned setup.
The surgeon is thus freed of this enormous burden and can implant the pedicle screws along the axis imposed by the topography of the pathological vertebra.
Regional sagittal vertebral statics are observed by virtue of a bending in one plane, aimed at restoring sagittal equilibrium.
Various mechanical solutions are proposed, particularly by successively fitting together elements which culminate in the securing of the screw/ball/rod triplet.
Geometrically complex recesses and the fitting-together of a series of elements allow the advantages of the above described screw/ball-jointed tulip element to be reproduced.
In spite of the considerable progress that this alternative represents, it is appropriate that a critical analysis be made of it, and this analysis can be summarized in three points:
1. The multi-axis U-shaped screws firstly do not allow rod/plate interchangeability, or if they do this entails disassembly rather akin to the "nesting Russian doll" principle. PA0 2. The use of successive spacing pieces may prove tricky, increasing the number of manoeuvres. PA0 3. The special-purpose instruments required involve just as many unknowns which add to the operating time, requiring medical auxiliaries training, and finally make maintenance more involved.
Furthermore, reduction of an anterolisthesis requires the use of screws with a U, the arms of which are extended backwards, at the expense of requiring far more space. Finally, in order not to stress the tightening elements during traction manoeuvres, use of a special-purpose reduction instrument is recommended but entails stressing the pedicle in tension; all of which cause preliminary weakening.
The mechanically reliable nature of the immobilization assumes a perfect fit, although such fit is uncertain under operating conditions (firstly the constraints imposed by the process, the interposition of tissue, poor visual inspection, etc.) where the implant is embedded.
The absence of rotational locking between the anchoring part and the multi-axis ball also makes dismantling difficult and sometimes impossible.